The present invention relates to spark timing control systems for internal combustion engines, and in particular to a method and apparatus of high precision spark timing control for all engine operations.
Various types of electronic ignition timing control systems have been developed for maximizing the thermal efficiency of internal combustion engines to meet increasing demands of energy savings.
Conventional systems employ a ferromagnetic toothed wheel having 90 teeth, for example, around the circumference thereof, the wheel being mounted on a pulley which is in turn coupled to the engine crankshaft for obtaining the necessary precision. Two electromagnetic pickup coils are provided to produce 180 pulses for each revolution of the crankshaft, the frequency of the pulses being multiplied by a waveform shaping circuit to 360 pulses per crankshaft revolution. A reference position signal is obtained in response to the piston of each engine cylinder reaching top dead center (TDC). This reference position signal is used to trigger a counter to initiate counting the engine speed-related pulses and provide an ignition pulse when this count value corresponds to an ignition target value.
Problems associated with prior art system include the limited space available for mounting the pair of pickup coils, the unreliable operation due to an increased number of electrical connections between the pickup coils and control circuitry, and the higher costs due to the complex signal processing circuitry.
According to another prior art approach, the reference position is advanced from the TDC point by a predetermined angle and a predicted spark timing is derived from the time elapsed from the reference position signal and ignition occurs after the lapse of a predetermined time.
This prior art method allows a simplified design for sensors and waveform shaping circuits with a consequential reduction in costs and increase in reliability. However, if the optimum advance angle is lagged substantially with respect to the reference pulse, the spark timing tends to deviate substantially during acceleration. More specifically, to obtain maximum thermal efficiency, conventional automotive engines have a maximum advance angle of 60 degrees before TDC. When the engine is accelerated rapidly as, for example, from an idling speed of 600 rpm to as high as 6000 rpm within a period of about 1 second, actual spark timing deviates as much as 12 degrees from the correct firing point.